What about Amaterasu (God of the Sun)? Itachi made it look like its WAY powerful than Tsukuyomi (God of the Moon)?
I'm talking about the general Mangekyou which is the next level in Sharingan, a prerequisite for the Tsukiyomi and Amaterasu, not the actual techniques themselves.

If thats his ultimate move than Gai would've beaten him since he doesn't look at the opponent's eyes when the opponent is a sharingan user. Remember Gai looks at the person feet when he fights a sharingan user
It's not as simple as that, he may be able to fight Itachi without looking at his eye's, but Itachi still has the upper hand as he can look at Gai, and fight him, when Gai has to look at his feet. Being able to disable/beat the ultimate move doesn't guarantee you beat your opponent. Itachi would have an arsenal of techniques at his disposal from all over the world.